1. Field of the Invention
The present invention relates to a lead propelling device capable of projecting a lead by a predetermined length from the leading end of the writing tool by knocking operation, and maintaining the projected lead by the predetermined length if knocking operation is repeated, and more particularly to an improvement of lead propelling device used in mechanical pencil, writing lead holder, or the like.
2. Description of the Related Art
A structure of a hitherto well-known mechanical pencil is shown in FIG. 36, and it is explained according to the drawing.
As shown in FIG. 36, a chuck 125 is provided at a front portion of a shaft tube 123 by way of a shaft joint 124, and a tightening tool 126 is fitted on the outer periphery of gripping part of this chuck 125. A biased chuck spring 128 is provided between a lead case 127 affixed to the rear end of the chuck 125 and the shaft joint 124. By this chuck spring 128, the chuck 125 is tightened by the tightening tool 126, and a lead 129 is gripped.
A slider 131 is fixed at the leading end of a tip 130 attached at the leading end of the shaft tube 123. The slider 131 has a leading end pipe 132 fixed at a front portion of a shaft 131a, and a holding chuck 133 made of rubber or other elastic material fixed at a rear portion, and the lead 129 projects from the leading end pipe 132 by penetrating through the holding chuck 133.
In this state, when the chuck 125 is advanced by pressing (knocking) a knock cover (not shown) fitted to the upper end of the lead case, since the chuck 125 is tightened by the tightening tool 126, it advances while gripping the lead 129.
As a result, the front end of the tightening tool 126 abuts against a step 130a of the tip 130, and when the chuck 125 further advances, tightening by the tightening tool 126 is cleared. Furthermore, if the chuck 125 advances, since the tightening tool 126 is engaged by the step 130a, only the chuck 125 moves forward, and the gripping part at the leading end of the chuck 125 is expanded.
When cleared from the knocked state, while the lead 129 is held in the holding chuck 133 and the gripping part of the chuck 125 is expanded, the chuck 125 retreats. The tightening tool 126 abuts against the shaft joint 124, and when the retreat of the tightening tool 126 is blocked, the chuck 125 begins to grip the lead 129.
At the end of knocking (initial state), the chuck 125 completes gripping of the lead 129.
Thus, in the conventional mechanical pencil, every time the knocking operation is repeated, the front end of the gripping tool 126 abuts against the step 130a to set in a state of expanding the gripping part of the chuck 125, and the front end of the tightening tool 126 abuts against the shaft joint 124 to set in a state of gripping the gripping part of the chuck 125, and that repeating these states, the lead 129 is propelled sequentially.
In such conventional mechanical pencil, since the lead is spent as writing continues, the lead is propelled by knocking occasionally. If forgetting to knock, the lead is worn up to the front end of the leading end pipe 132, and writing may be scratchy, or finally the writing paper may be torn by the front end of the rigid leading end pipe.
On the other hand, if knocked more than necessary to propel the lead, the projected lead may be too long, and the lead is often broken. Or the lead may be projected more than necessary by knocking unknowingly while writing.
As means for solving such problems, Japanese Utility Model Application Laid-open No. S55-99887 discloses a writing lead holder capable of projecting the lead by a specific length from the leading end of the writing tool by knocking operation and maintaining a specific length of a projected lead if the knocking operation is repeated.
This writing lead holder is explained with reference to FIGS. 37 to 40.
As shown in the figures, at the leading end of the shaft tube 151, a taper hole 152 is formed for fitting the chuck 153 for gripping the lead. Inside of the shaft tube 151, there is a tubular lead case 154 for accommodating a lead 155.
An annular protrusion 156 is formed nearly in an intermediate position of the chuck 153. A coil spring 157 is loaded between the annular protrusion 156 and the lead case 154, and the upper end of the chuck 153 is slidably attached in a hole 154a provided in the lower part of the lead case 154. A coil spring 158 is loaded between the annular protrusion 156 and the upper surface of the shaft tube 151 (taper hole 152). The coil spring 158 has a stronger spring than the coil spring 157.
A support arm 159 is affixed to the periphery of the lower part of the lead case 154. A stopper 160 fitting to the leading end of the chuck 153 is formed at the leading end of the support arm 159.
Explaining the operation of the writing lead holder having such structure, first, by removing the cap 161, the writing lead 155 is put into the lead case 154 and chuck 153, and the cap 161 is put on (see FIG. 37). Then, by knocking the cap 161, the lead case 154 compresses the coil spring 157, and moves downward. Along with this move, the support arm 159 also moves downward, and the stopper 160 is departed from the chuck 153 (see FIG. 38). At this time, the upper end of the chuck 153 slides in the hole 154a of the lead case 154.
By further knocking the cap 161, the lead case 154 compresses the coil spring 158, and moves downward. Along with this move, the leading end (gripping part) of the chuck 153 is expanded, and the lead 155 falls by its own weight to the stopper 160 (see FIG. 39).
Later, when knocking of the cap 161 is cleared (released), the chuck 153 and support arm 159 restore the initial state by the repulsive force of the coil springs 157, 158. At this time, the chuck 153 is closed by fitting of the taper hole 152, and the lead 155 is gripped. Since the lead 155 is held by the chuck 153, the lead 155 is projected from the stopper 160 by a predetermined length (see FIG. 40).
When the cap 161 is knocked again, since the lead 155 is held by the chuck 153, only the support arm moves downward, and the state becomes as shown in FIG. 39, and when released from knocking, the state becomes as shown in FIG. 40.
Thus, in this proposed writing lead holder, by knocking operation, the lead can be projected from the leading end of the writing tool by a predetermined length, and the projected lead can be maintained at a predetermined length if knocking operation is repeated.
However, since the coil spring 157 biasing the stopper 160 backward and the coil spring 158 biasing the chuck 153 backward are arranged in series across the annular protrusion 156 of the chuck 153, the projecting length of the lead 155 may not be stable.
For example, if the compression of the coil spring 157 biasing the stopper 160 is advanced to increase the biasing force until becoming equal to the initial biasing force of the spring 158 biasing the chuck 153, the chuck 153 begins to move, and expansion of the chuck 153 starts.
That is, by sliding resistance between members or other factor, the expanding timing of the chuck 153 may be delayed, and the lead projecting extent may not be stable.
Besides, the coil spring 157 biasing the stopper 160 continues to be compressed even after start of expansion of the chuck 153, and the stopper 160 continues to advance. As a result, the distance between the stopper 160 for determining the projection of the lead 155 and the leading end of the chuck 153 continues to increase, and the lead projecting amount may not be stable.
In this action, the relation of the biasing force of the coil springs is expressed as x1<y1, x2=y1, where x1 is the initial biasing force of the coil spring 157 biasing the stopper 160 backward, x2 is the biasing force when the chuck 153 begins to expand, and y1 is the initial biasing force of the coil spring 158 biasing the chuck 153 backward.
The coil spring 158 for biasing the chuck 153 backward is regulated by the rule as mentioned above in relation to the biasing force of the coil spring 157 for biasing the stopper 160 backward (the coil spring 158 is required to have a stronger biasing force), and, for example, if the lead is very fine in a mechanical pencil, the lead may be broken or damaged by the chuck, and it may be bit (broken) and the surface may be roughened (damaged).
In other action, in the case the chuck 153 begins to be expanded when the coil spring 157 biasing the stopper 160 is fully compressed, the overall length of the coil spring 157 when compressed fully may be changed easily due to fluctuations of wire rod diameter, fluctuations of number of turns, or deviation in the radial direction when winding the coil tightly.
In this action, the relation of the biasing force of the coil springs is expressed as X1<Y1 and X2<Y1, where X1 is the initial biasing force of the coil spring 157 biasing the stopper 160 backward, X2 is the biasing force when compressed fully, and Y1 is the initial biasing force of the coil spring 158 biasing the chuck 153 backward.
In this action, too, the coil spring 158 for biasing the chuck 153 backward is regulated by the rule as mentioned above in relation to the biasing force of the coil spring 157 for biasing the stopper 160 backward (the coil spring 158 is required to have a stronger biasing force), and, for example, if the lead is very fine as mentioned above, the lead 155 may be bit by the chuck 153, and the surface may be roughened.
In this writing lead holder, members very difficult to machine are used, which is also a bottleneck for realizing this proposal.
For example, as for the shaft tube 151, aside from forming a taper hole 152 at the leading end, through-holes 151a (see FIG. 38) for pass-through of the support arm 159 of the stopper 160 must be formed at two positions, and this shape is very difficult to process, the productivity is poor, and it is predicted to be a very expensive component.
The support arm 159 requires a very long distance up to the leading end of the lead case 154, and since it is passed in the shaft tube 151, it is predicted that the thickness and width may not be assured sufficiently. The stopper 160 is formed integrally in the support arm 159 but such support arm is very difficult to process, the productivity is poor, and it is predicted to be a very expensive component.
It is also difficult to assemble the writing lead holder. For example, the coil spring 158 biasing the chuck 153 backward is placed between the annular protrusion 156 provided in the center of the chuck 153 and the leading end inner side of the shaft tube 151, but it is impossible to assemble as far as estimated.
Aside from such inconvenience, this proposal seems to project a thick lead as the name of writing lead holder suggests, and this structure is not suited to a mechanical pencil using a fine lead. In a mechanical pencil, for example, since the lead is fine, and in order to prevent lead from breakage, the leading end for projecting the lead and the chuck must be disposed so as not to cause misalignment while maintaining enough rigidity To prevent lead breakage, the projecting length of lead is set to be short, and the leading end of the writing tool must be thinly tapered so that the leading end may be easily visible while writing.